1. Field of the Invention
The present invention relates to a serial transmission system. In particular, the present invention relates to a serial transmission system which transmits a digital audio signal via a serial transmission path.
2. Description of Related Art
In recent years, a digital audio technology has advanced conspicuously and a so-called multimedia device such as a DVD (Digital Versatile Disc) video and the like has come into wide use in household. Moreover, a multi-channel audio signal that uses more channels than the conventional two-channel stereo audio signal has come to be used as an application of the multimedia device. Accordingly, the number of transmission lines of the audio signal is increased and thus terminals of an LSI and the number of interconnections formed on a substrate are also increased. In a case of a general serial transmission, for example, the two-channel audio signal is transmitted by a total of three signals: a transmission clock, an LR clock for distinguishing between the left channel and the right channel of the two-channel stereo audio signal, and data. In order to transmit a six-channel audio signal by the same method, additional two signals are required and a total of five interconnections are necessary.
Furthermore, as the number of types of recording media such as the DVD, a CD (Compact Disc) and the like increases and a digital broadcasting becomes common, a plurality of sampling frequencies of the audio signal have come to be used. However, this causes a long silence period at a time when the sampling frequency is changed and a partial sound vanish after the sampling frequency is changed.
Both of the frequency of 48 kHz used by the DVD video and the frequency of 44.1 kHz used by the CD are the primary sampling frequencies, and many existing audio signals employ the two frequencies.
It is therefore necessary, for example, to transmit an audio signal whose sampling frequency is 48 kHz and then transmit an audio signal whose sampling frequency is 44.1 kHz by using the same transmission path.
In this case, it is necessary to once stop the transmission of the sampling frequency of 48 kHz, initialize a transmission device, a receiving device and the transmission path, change the setting to that of the sampling frequency of 44.1 kHz, and thereafter start the transmission of the sampling frequency of 44.1 kHz. Therefore, the silence occurs during a period from the stop of the transmission to the start of the next transmission, because the audio signal can not be transmitted. In addition, the sampling frequencies of 48 kHz and 32 kHz are mainly used in the digital broadcasting. For example, let us consider a case where the audio signal changes in the sampling frequency from 48 kHz to 32 kHz. In this case, if the audio signal whose sampling frequency is 32 kHz is transmitted after the transmission device, the receiving device and the transmission path are initialized, the head of the audio signal of the sampling frequency of 32 kHz may not be output, because it takes a long time for initializing the transmission path and hence the head does not make it in time for the output.
As a related technique, Japanese Laid-Open Patent Application JP-P2004-147047 discloses a serial transmission device which transmits a multi-channel audio signal. The audio data transmission system disclosed in the patent document is provided with an audio processing chip and a processor. The audio processing chip receives audio data of a plurality of channels and serially outputs the received audio data with respect to each channel. The processor performs signal processing of the audio data transmitted from the audio processing chip. The audio processing chip includes a synchronization signal generation unit which generates a synchronization signal maintaining a first logic level for a predetermined length depending on a channel number of the audio data to be transmitted to the processor. The processor is characterized by including a channel determination unit which measures the length for which the synchronization signal output from the synchronization signal generation unit is maintained at the first logic level to determine the channel number of the audio data.
FIG. 1 is a timing chart showing an operation of the transmission device, the serial transmission path and the receiving device according to the related technique. Shown in FIG. 1 are an operation clock signal (CLK) 5-100, a data line signal (DATA) 5-102, and a synchronization signal (SYNC) 5-101 indicating a synchronization timing of data input-output time of the DATA 5-102 and a selected channel. As shown in FIG. 1, the transmission device reflects the channel number to a pulse width of the SYNC 5-101 from timing (a). That is to say, the SYNC 5-101 has a pulse width of one clock at a time when the data of a channel-1 is transmitted and has a pulse width of four clocks at a time when the data of a channel-4 is transmitted. The receiving device recognizes the SYNC 5-101 to identify the channel number and then receives the DATA 5-102 from timing (b).
According to the configuration, it is possible to increase the number of channels of the data by increasing choices of the pulse width of the SYNC 5-101. Therefore, there is no need to increase the transmission line in addition to the three lines the CLK 5-100, the SYNC5-101 and the DATA5-102, which can solve the problem of the increase in the number of interconnections. However, there is no means for transmitting the sampling frequency, and thus only the audio data of a predetermined fixed frequency can be transmitted between the transmission device and the receiving device. In other words, if the audio signals of different sampling frequencies are to be transmitted, the silence period occurs and a synchronization relation between an image and the audio is broken.
The inventor of the present application has recognized the following point. In the above-described serial transmission, the sampling frequency that is an important attribute of the digital audio signal is not transmitted concurrently with the multi-channel audio signal. As a result, the long silence period and the phenomenon that the head of the audio signal can not make it in time for the output are caused.